Autoradiographic image intensification is a process wherein the silver or other imaging material in a developed and fixed film or plate is converted to a radioactive compound and an intensified image is obtained on a receiver emulsion which is exposed by radiation from the activated film.
The autoradiograph reproduces the original image with increased density and contrast so that, in many cases, images which were invisible on the original plate or film are visible on the receiver emulsion. Autoradiography is used to obtain jan improved image from underexposed or underdeveloped films or plates, to obtain better images and more information from relatively less dense sections of a properly exposed and developed film or plate, to obtain a satisfactory image of an aged or faded film and to generally extend the limits of photographic sensitivity.
Films or plates which are amenable to autoradiographic intensification may seem to contain little or no useful information when examined visually or by photometric measurements. The information is stored in developed silver grains, the number of which is too small for appreciable visual or photometric detection. The number of silver grains may be smaller than that in a normal developed film because the film was accidentally underexposed or underdeveloped or because there was simply not enough light intensity available (e.g. in astronomy). Autoradiographic intensification retrieves the information stored in the low density silver grains by reproducing the original image with increased silver density, proportional to the original density, on the receiver film.
Autoradiography is described by M. Thackray in British Pat. No. 1,394,664 and "Autoradiography of Radioactive Photographic Images -- Applications to Intensification, Restoration, Precision Etching, Photomechanical Reproduction and Photographic Research," Australian Atomic Energy Commission AAEC/E 317, ISBN 0 642 99656 3, September, 1974, and in various other references, including Dement, U.S. Pat. No. 2,603,775; and Hermann et al, East German Pat. No. 66559. The existing radiographic methods, however, have exhibited one or more serious shortcomings.
Prior chemical activation methods which use the beta-emitting isotope, sulfur-35, are all based on toning chemistry and have numerous disadvantages. The use of ionic sulfur-35 compounds to form radioactive silver sulfide is required. One modification of the method is to treat the original film with a solution of sodium or potassium sulfide or polysulfide ions labeled with the radioactive isotope, sulfur-35. Alternatively, the image silver is bleached to ionic silver halide and then treated with radioactive sodium sulfide-S35 solution. (See Thrackey, supra, Dement, Supra, and Hermann et al, supra.
Previously described methods for using sulfur-35 to make the image radioactive have disadvantages which severely limit (1) the extent to which the process can be used to intensify images, (2) the convenience with which the process can be used in an ordinary photographic laboratory, and (3) the practicality of adapting the process to batch or continuous processing.
The amount of intensification, i.e., increase in image-density to fog-density ratio of the autoradiograph as compared to the original negative, is severely limited because sulfide ions react with gelatin of the negative or positive as well as with photographic silver. The receiver film of the autoradiograph responds indiscriminately to radiation from silver and from gelatin. In areas of low silver density, as much or more radiation may be emitted from the gelatin as from the image silver.
Thackray, in British Pat. No. 1,394,664, described a method of rinsing the activated film with inactive sulfide solution, so that inactive sulfur ions exchange with sulfur compounds in the gelatin more rapidly than with the silver sulfide of the image to produce selective reduction of activity in the gelatin. This rinse results in some improvement in silver-to-gelatin activity ratio but does not eliminate the problem of extraneous radiation from S35 exchanged with the gelatin in the emulsion and also causes significant losses of activity from the silver. The method is difficult to use because the rinse time must be estimated for each type of original film to avoid removal of activity from the image silver. The deficiencies of this rinsing method are particularly apparent in attempts to enhance images in areas of very low densities.
Polysulfide ions also produce undesirable background activity because of the tendency to form colloidal solutions, which precipitate into the gelatin. A similar problem is encountered with the sodium sulfide method because during the bleach step some of the image silver is converted to colloidal silver sulfide, which settles out in the gelatin and produces undesirable background activity.
Therefore, prior art sulfur-35 image enhancement techniques are severly limited as to signal-to-noise increase or may even show a decrease in signal-to-noise ratio, thus producing the opposite of intensification.
The foregoing processes are also very sensitive to the pH of the radiotoning solutions, which must be controlled very carefully. This can be difficult and time consuming. With the polysulfide solutions, exposure to air oxidation also must be prevented.
Moreover, the sulfur-35 compounds used heretofore are chemically unstable and decompose rapidly to cause waste and inconvenience. The specific activity of the solutions must be kept relatively low to minimize radiolytic oxidation, but when low specific activity solutions are used for activation, the contact time to produce the autoradiograph is increased by hours or days. These solutions also give off highly toxic hydrogen sulfide gas. When the solutions contain radioactive sulfur, the gas evolved is radioactive. In the case of sodium sulfide, a primary decomposition product is sodium thiosulfate, which is a solvent for silver halide.
Any activating process which employs a bleach step has the disadvantage that the bleach time is judged by observing the color of the film as bleaching progresses. This is an empirical procedure requiring skill and experience and is difficult, if not impossible, to automate.
Thiourea has been used in photographic chemistry in several ways, but the use of thiourea - S.sup.35, described in this application, is a new technique with different chemistry and for a different purpose. Thiourea has previously been used as a toner. (See C. E. Mees, Theory of the Photographic Process, Second Edition, 1954). The toning chemistry requires photographic silver to be converted to silver halide before adding an alkaline thiourea solution to convert the image to silver sulfide. Any intensification which occurs with the toning process is due to the color of the silver sulfide image as compared to the elemental silver image. Any intensification achieved in this way is very small compared to the very high level of intensification achieved with autoradiography due to the photographic effect of beta particles (from the thiourea - S.sup.35) on the autoradiograph emulsion. Thackray, Supra, discloses use of thiourea in acidic solution as a silver sulfide solvent, which is also an entirely different application from the one described here.
Isotopes other than sulfur-35 all have one or more of the above disadvantages and additional objectional features, including high biological toxicity, undesirable gamma radiation which fogs nearby film and presents a health hazard, inconveniently long or short half-lives and relatively high cost. (See Thackray, AAEC/E 317, Appendix A for more detail.)
Autoradiographic image intensification by any of the prior art methods is a complicated procedure requiring some chemical skill and not easily adapted to ordinary photographic laboratories.
Nuclear reaction methods of making films radioactive, (E.G., neutron activation), are limited in usefulness by the specialized equipment required to produce the neutron or other radiation. Additionally, these methods usually activate impurity elements in the film as well as silver. Thus, background fog is increased and intensification is limited.